This February 2016 as you all know has 29 days – one extra unpaid day of work for people who get paid on a monthly or bi-monthly cycle. Leap years come every 4 years but not always……..
As we all know every 4th year is a leap year, one where February has one extra day – 29 instead of 28. This is because the earth rotates around the sun in 365 and one quarter days. So to account for this every 4th year (one that is divisible by 4) is deemed to be a leap year.
The history of different calendars is fascinating – there are calendars based on cycles:
- lunar (the most obvious from a historical perspective),
- solar (by analyzing the position of the zodiac signs – the way the sun moves across the sky over the course of a year),
- lunisolar (a lunar calendar that occasionally adds one intercalary month to remain synchronized with the solar year over the long term, and formed the basis for our current calendar)
- etc. that are wonderfully elucidated in Wikis (Calendar Wiki)
- and more famously, the Mayan calendar that predicted the end of days on December 21, 2012 due to the end of the long count calculation
Over the years to account for discrepancies, calendars have undergone reforms. Calendar reform, properly called calendrical reform, is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar.
The Julian Calendar has a regular year of 365 days divided into 12 months. A leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long. It was intended to approximate the tropical (solar) year. This is what we are taught in school…..
But this is not God’s world and things are not exact. So instead of being 365.25 the actual number is 365.2425….. Although Greek astronomers had known, at least since Hipparchus, a century before the Julian reform, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the year gained about three days every four centuries compared to observed equinox times and the seasons.
The Gregorian Calendar corrected the issue with the Julian calendar by assuming the year to be 365.2425 (365d, 5h, 49m, 12s) long. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, years evenly divisible by 100 are not leap years, except that years evenly divisible by 400 remain leap years. Thus:
- 2100, 2200, 2300 are not leap years
- 2000, 2400, 2800 are leap years
When the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days. The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).
So does that mean that 1900 was really not a leap year? Well yes and no….
Issues with adoption
Although Gregory’s reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church and the Papal States. The changes that he was proposing were changes to the civil calendar, over which he had no authority. They required adoption by the civil authorities in each country to have legal effect. So every county took it own time to adapt to the new calendar.
By the time Great Britain adopted this system in 1752, they were behind by 11 days and when the dates changed, there were a large number of protests: “Give us back our 11 days” was the chant by the labor class that got paid by the hour, but had to pay rent by the month.
The Russian Revolution, traditionally known as Red October, the Bolshevik Revolution or simply The October Revolution took place on October 25, 2017. But this was before the Russian adoption of the new calendaring system and the date now falls on November 7, 1917. So the October Revolution is actually celebrated in November….
…. and Microsoft Outlook still classifies 1900 as a leap year.
So depending upon whether you were in Russia, or use Microsoft Outlook – 1900 may or may not have been a leap year!
But 365.2425 is still not exact (365.242375 is more exact), and a further correction is necessary. So here is the formula:
“Every year multiple of 4 is a leap year.
Every 100th year is not a leap year except if it is divisible by 400
Every 1000th year is a leap year except if is divisible by 4000.”
- 2016 is a leap year
- 2100 will not be a leap year
- 3000 will be a leap year
- 4000 will not be a leap year… that is, if civilization lasts that long, my guess is that we will move to Mars and adopt a calendar based on the Martian year!